tools/Makefile: fix location of the tree root for .gitconfig

The tree root is one level above tools/, not two.

tools/Makefile

@@ -48,7 +48,7 @@ spotless: clean
 
 .PHONY: gnu.src
 gnu.src:
-	if [ -f ../../.gitmodules ] && \
+	if [ -f ../.gitmodules ] && \
 	   git submodule status --recursive | grep -q '^[+-]'; then \
 		git submodule update --init --recursive ; \
 		rm -rf gnu gnu.*.stamp ; \